This invention relates to a method of purifying cyclic dichlorophosphazenes containing trace amounts of protic impurities. More particularly, the invention relates to a method of purifying cyclic dichlorophosphazenes containing protic impurities by treatment with boron trihalides under specified temperature and pressure conditions (described hereinafter).
Chlorophosphazenes of the formula (NPCl.sub.2).sub.n, wherein n is 3 to 9, are conventionally prepared by the reaction of phosphorus pentachloride and ammonium chloride. Chlorophosphazenes prepared by the above reaction are in fact a mixture of compounds consisting of cyclic dichlorophosphazene oligomers such as the cyclic trimer (NPCl.sub.2).sub.3 and tetramer (NPCl.sub.2).sub.4, low molecular weight oily linear oligomers and certain other unspecified impurities. Of these compounds, the cyclic trimer is the preferred compound for use as the polymerizable monomer in the preparation of high molecular weight linear polydichlorophosphazene polymers, although mixtures containing the cyclic trimer and minor amounts of tetramer are also suitable.
The general approach of prior art methods of purifying the chlorophosphazenes produced by the reaction of phosphorus pentachloride and ammonium chloride has involved the separation of the cyclic oligomers from the linear oligomers and other impurities either by the utilization of the differences in the boiling points of these compounds or by the different reactivities of these compounds with aqueous bases. Specific purification methods have involved the extraction of chlorophosphazene from petroleum ether solutions with sulfuric acid, U.S. Pat. No. 3,008,799; controlled crystallization in a variety of solvents, U.S. Pat. No. 3,378,353; separation of the trimer and tetramer from the produced mixtures through distillation involving a spinning band column, U.S. Pat. No. 3,379,510; contacting molten chlorophosphazenes with an inert solvent vapor so as to selectively vaporize the cyclic trimer, separating a solvent vapor phase laden with trimer and some tetramer from the molten residue, condensing it to form a solution of trimer and tetramer in the solvent and subsequently recovering trimer together with some tetramer from the solution; U.S. Pat. No. 3,677,720 steam distillation of chlorophosphazenes resulting in hydrolysis of x=4-9 and hence separation of the trimer. Chemical Abstracts, Volume 77, Page 540, 159648D (1972); saponification and hydrolysis of chlorophosphazenes by treatment with aqueous sodium or ammonium hydroxide resulting in unreacted trimer and tetramer, U.S. Pat. No. 3,694,171; contacting a crude chlorophosphazene with a Bronsted base, removing water formed from the reaction of the Bronsted base with the impurities in the chlorophosphazene, and then recovering at least a very high purity cyclic chlorophosphazene trimer, U.S. Pat. No. 3,952,086; and treating the crude chlorophosphazene by first subjecting it to at least two water-washings and then further purifying it by a conventional purification procedure such as recrystallization, sublimation, distillation or melt-filtering, U.S. Pat. No. 4,175,113.
The aforementioned prior art approaches to the purification of chlorophosphazenes in many instances result in the effective separation of the cyclic oligomers from the linear oligomers and other nonprotic impurities. However, none of these prior art purification methods are effective enough to remove trace amounts of protic impurities which are present in cyclic oligomers.
We have found that the presence of trace amounts of protic impurities in cyclic dichlorophosphazene oligomers, e.g., (NPCl.sub.2).sub.3, (NPCl.sub.2).sub.4, employed as the starting materials for the preparation of high molecular weight polydichlorophosphazene polymers inhibits the polymerization of such oligomers. Accordingly, a process for removing protic impurities from such cyclic oligomers would be highly desirable.
We have now discovered an effective and simple process for removing protic impurities from cyclic dichlorophosphazene oligomers. The method involves the treatment of the oligomer with boron trihalides under appropriate conditions (described below) prior to the polymerization procedure.
Boron trihalides have previously been employed as catalysts in the polymerization of cyclic dichlorophosphazene oligomers as illustrated by U.S. Pat. Nos. 4,116,891 and 4,123,503. However, insofar as applicants are aware, such compounds have not been employed to pretreat the oligomer prior to polymerization in order to remove protic impurities as is the case in the present invention.